This invention relates to digital motor speed controls of a type similar to those disclosed in the following U.S. patents: Haner et al. U.S. Pat. No. 3,543,116; Jones U.S. Pat. No. 3,110,853; Loyd U.S. Pat. No. 3,546,553; Gifft U.S. Pat. No. 3,176,208; and Strand et al. U.S. Pat. No. 3,331,006. These patents disclose several circuits in which the speed of an electric motor is controlled through the use of digital circuits to approach a preset desired speed value. In these circuits, the actual speed of the motor is measured by means of a tachometer pulse generator coupled to the motor shaft. An adjustable frequency oscillator is preset to a frequency which is proportional to a desired speed value for the motor. The frequency of the tachometer pulse generator output is compared to the preset frequency in a digital frequency comparison circuit. If the tachometer pulse generator frequency is lower than the preset frequency, the motor is speeded up, and if the tachometer pulse generator frequency is above the preset frequency, the motor is slowed down. The speed measurement and speed comparison functions are continuous and occur concurrently with the speed correction.
However, in accordance with this invention, it has been found that the speed measurement, speed comparison, and speed control circuits can be greatly simplified by measuring the motor speed and determining whether it is fast or slow in a first fixed time interval and then correcting the motor speed in a second fixed time interval which follows the first time interval. This time division between speed measurement and speed correction eliminates the need for an adjustable oscillator and frequency comparison circuit and also significantly simplifies the speed correction circuit. This simplification of the circuit provides a substantial reduction in cost and increase in reliability.